Bump next from 14.2.15 to 14.2.21

Bumps [next](https://github.com/vercel/next.js) from 14.2.15 to 14.2.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](https://github.com/vercel/next.js/compare/v14.2.15...v14.2.21)

---
updated-dependencies:
- dependency-name: next
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -8,7 +8,7 @@
     "start": "pnpm dlx serve out"
   },
   "dependencies": {
-    "next": "^14.2.15",
+    "next": "^14.2.21",
     "nextra": "^3.0.15",
     "nextra-theme-docs": "^3.0.15",
     "react": "^18.3.1",